US-funded network

Tor was thrust into the spotlight in the wake of controversy resulting from leaks about the National Security Agency and other cyberspy agencies. Edward Snowden, the whistleblower who revealed the internal memos and who now has asylum in Russia, uses a version of Tor software to communicate.

Documents released by Mr Snowden allege that the NSA and the UK's GCHQ had repeatedly tried to crack anonymity on the Tor network.

Tor was originally set up by the US Naval Research Laboratory and is used be people who want to send information over the internet without being tracked.

It is used by journalists and law enforcement officers, but has also been linked to illegal activity including drug deals and the sale of child abuse images.

In its 2013 financial statements, the Tor Project - a group of developers that maintain tools used to access Tor - confirmed that the US Department of Defense remained one its biggest backers.

The DoD sent $830,000 (£489,000) to the group through SRI International, which describes itself as an independent non-profit research centre, last year.